The disclosures of the following priority application is herein incorporated by reference:
Japanese Patent Application No. 11-343694 filed Dec. 2, 1999.
1. Field of the Invention
The present invention relates to a wide-angle lens and, in particular, relates to a super wide-angle lens having a large relative-aperture and a large angle of view.
2. Description of Related Art
In conventional projection method, when the image height is denoted by y, the focal length of a lens is denoted by f, and an angle formed by an object measured from the optical axis is denoted by xcex8, the expression y=fxc2x7tan xcex8 is satisfied. In this method, although a proposal of a super wide-angle lens having the inclusive angle (angle of view) of 2 xcfx89=110xc2x0 or more has been rare, and a proposal of a super wide-angle lens having the relative-aperture of F 3.5 or less has been very rare, lenses proposed by, for example, Japanese Patent Application No. 9-113798 and by Japanese Patent Application No. 9-113800 filed by the same assignee of the present application have been known.
Moreover, although a proposal of a super wide-angle lens having the inclusive angle of 2 xcfx89=115xc2x0 or more with the conventional projection method, which satisfies y=fxc2x7tan xcex8, and having the relative-aperture of F 3.5 or less has been very rare, Japanese Patent Application No. 10-325923 filed by the same assignee of the present application, for example, has proposed a lens having the inclusive angle of 2 xcfx89=115xc2x0 or more and the relative-aperture of F 3.5 or less.
However, a super wide-angle lens having the inclusive angle of 2 xcfx89=115xc2x0 or more with the conventional projection method, which satisfies y=fxc2x7tan xcex8, and having the relative-aperture of about F 2.8 has not been proposed, and not been commercialized. In this situation with the present inventor""s exertion and diligence, an optical system is obtained that has the inclusive angle (angle of view) of 2 xcfx89=118xc2x0 or more with the conventional projection method, which is almost limit with the projection method and has not existed, and also has a large relative-aperture of F 2.8.
Problems for developing such an optical system are compactness to be able to use regularly, high optical performance, securing a peripheral quantity of light, and productivity of an aspherical surface. It is particularly important whether an aspherical lens to be used can be produced by using current mass production technique or not. In considering manufacturing method, instead of using a fine ground aspherical surface, which has not good productivity, to be able to use a glass molding method, which has high productivity, is directly connected to reducing costs, and is even advantageous to the user. In considering from this point of view, the aforementioned Japanese Patent Application No. 9-113798 or Japanese Patent Application No. 9-113800 proposed an optical system having a field of view of 2 xcfx89=105.6xc2x0 and the relative-aperture of F 2.87. However, aspherical lenses proposed in these optical systems have been difficult to be manufactured by using a fine ground method or a glass molding method and, as a result, productivity has been low. The angle of view has been only about 105xc2x0, so that it has been insufficient. If the angle of view is made wider without taking measures, then the aspherical lens will become even more difficult to be manufactured.
Japanese Patent Application No. 10-325923 proposed a large relative-aperture super wide-angle lens having the angle of view of 2 xcfx89=118xc2x0 and the relative-aperture of F 2.9. However, an aspherical lens located to an object side of the lens system was more difficult to be manufactured than the optical system described above, so that the productivity of the lens system was low. Moreover, the optical performance was insufficient, focusing method had a disadvantage, and the size was large.
The present invention is made in view of the aforementioned problem and has an object to provide a large relative-aperture super wide-angle lens having the inclusive angle (angle of view) of 2 xcfx89=115xc2x0 or more with the conventional projection method, which satisfies y=fxc2x7tan xcex8, having the relative-aperture of about F 2.8, having compactness and high optical performance, and having small variation of aberration in relatively closed object distance.
In order to solve the problems described above, the present invention has, in order from the object, a divergent lens group having negative refractive power, and a convergent lens group having positive refractive power. The divergent lens system includes at least one negative aspherical lens element, and the aspherical lens satisfies the following conditional expressions (1) and (2):                     0         less than                                             (                                                d                                      φ                    ⁢                                          xe2x80x83                                        ⁢                    max                                                  -                                  d                  0                                            )                        /                          h              max                                                          (                                                d                  30                                -                                  d                  0                                            )                        /                          h              30                                       less than         3                            (        1        )            xe2x80x83xe2x88x9215 less than fasp/f0 less than xe2x88x921.7xe2x80x83xe2x80x83(2)
where d0 denotes the thickness of the aspherical lens along the optical axis (center thickness), dxcfx86max denotes the thickness of the aspherical lens parallel to the optical axis at the height of the maximum effective aperture on the image side surfacer d30 denotes the thickness of the aspherical lens parallel to the optical axis at the height of 30% of the maximum effective aperture when the maximum effective aperture on the image side surface is assumed to be is 100%, hmax denotes the maximum effective radius of the aspherical lens on the image side surface, h30 denotes the 30% of the maximum effective radius of the aspherical lens when the maximum effective aperture on the image side surface is assumed to be 100%, fasp denotes the paraxial focal length of the aspherical lens, and f0 denotes the focal length of the super wide-angle lens.
In the present invention, there are at least two aspherical surfaces in the divergent lens group, and the most object side aspherical surface preferably has a third order term satisfied with conditional expression (3) when the aspherical surface is expressed by the following aspherical expression (A):
X(y)=(y2/r)/[1+(1xe2x88x92xcexaxc2x7y2/r2)1/2]+C2xc2x7y2+C3xc2x7|y|3+C4xc2x7y4+C6xc2x7y6C8xc2x7y8+xc2x7y8+C10xc2x7y10+C12xc2x7y12+C14xc2x7y14+C16xc2x7y16xe2x80x83xe2x80x83(A)
1xc3x9710xe2x88x926 less than |C3| less than 1xc3x9710xe2x88x922xe2x80x83xe2x80x83(3)
where X(y) denotes the distance (sag amount) between the tangent plane to the vertex of the aspherical surface and the aspherical surface along the optical axis at the height y vertical from the optical axis, r denotes the reference radius of curvature, xcexa denotes a conical coefficient, and Cn denotes the aspherical coefficient of n-th order.
In the present invention, the super wide-angle lens having the inclusive angle of the lens system of 2 xcfx89=110xc2x0 or more includes, in order from the object, a divergent lens group having negative refractive power, and a convergent lens group having positive refractive power. The divergent lens group includes, in order from the object, a negative meniscus lens having a convex surface facing to the object side, and a negative aspherical lens having at least one surface located to the image side with an aspherical shape that the radius of curvature becomes weak from the optical axis to the periphery. The aspherical lens satisfies the aforementioned conditional expression (1).
Moreover, the present invention has at least one focusing lens group for focusing to a near-distant object in the convergent lens group having positive refractive power, and the following conditional expression (4) is preferably satisfied:
3 less than fx/f0 less than 12xe2x80x83xe2x80x83(4)
where fx denotes the focal length of the focusing lens group, or that having the shortest focal length when a plurality of focusing lens groups are there, and f0 denotes the focal length of the whole system of the super wide-angle lens.
Furthermore, the present invention provides a photographing apparatus using the aforementioned super wide-angle lens.